Just six donors accounted for all of the $375,000 raised by the group between its inception on July 15 and July 24, the period covered in the pre-primary campaign finance report it was required to file with the Federal Election Commission.

Booker faces Democratic U.S. Reps. Frank Pallone and Rush Holt and Assembly Speaker Sheila Oliver in a special primary, called to fill the seat of the late Democratic Sen. Frank Lautenberg, who died on June 3. A recent Quinnipiac poll shows Booker with 54 percent of the vote and no challenger drawing more than 17 percent.

The Mobilization Project received $100,000 from Seth Klarman, CEO of the Baupost Group, a Massachusetts-based hedge fund. Ravenel B. Curry III and Ravenel B. Curry IV, both investors with New York-based hedge fund Eagle Capital Management, each also chipped in $50,000.

Other donations included $100,000 from Michael Fux, CEO of Comfort Revolution, which makes mattress pads, and $50,000 from Andrew Tisch, the chairman of Loews Corp., a holding company whose assets include CNA Financial, an offshore drilling subsidiary and a hotel chain.

A Center for Public Integrity review of FEC filings indicates that four of the Mobilization Project’s contributors — Fux, Curry IV, and both Tisches — have also given to directly Booker’s Senate campaign. And all but the older Curry have contributed to Booker’s leadership PAC since 2011 — giving a combined $40,000, according to the Center for Responsive Politics.

Loews maintains a significant presence in Washington. Over the past five years, it has spent about $2 million annually lobbying the federal government on a range of issues, including taxes, finance and energy regulation.

While super PACs ordinarily use their funds for television and radio advertisements, the Mobilization Project has focused all of its resources on a pro-Booker ground game — canvassing operations, literature and telephone calls.

Two other groups have been active in the New Jersey race — Pac Plus and the American Commitment Action Fund.

Pac Plus — which can act as both a super PAC and as a regular PAC, making direct contributions to candidates — backs Booker and has spent a little more than $41,000 on advertisements in the race so far. American Commitment Action Fund has spent $32,000 since late July as part of an ongoing online advertising campaign opposing Booker’s Senate bid.

American Commitment Action is headed by Phil Kerpen, the former vice president for policy at Americans for Prosperity, a conservative nonprofit with ties to billionaire brothers Charles and David Koch.

Primary Source dove into developments in the post-Citizens United world of money in politics. It featured original, entrepreneurial reporting by Center staff, as well as examinations of primary source documents.